1. Field of the Invention
The present invention generally relates to a communication apparatus and, more particularly, is directed to a radio communication apparatus suitably to be applied to a cordless telephone connected to a digital line network such as an ISDN (an Integrated Services Digital Network) and so on.
2. Description of the Related Art
Recently, there have been utilized various types of cordless telephones which are movable freely and able to perform a communication within a predetermined service area, e.g., a radius of about 100 m. The cordless telephone is constituted by a master station directly connected to a telephone line network and a remote station carried by a user so that a radio communication is performed between the master station and the remote station. Some types of the cordless telephones can provide a plurality of remote stations which are able to communicate with a single master station.
FIG. 1 shows an arrangement of an example of conventional cordless telephones connected to a general telephone line network. In this example, three remote stations 7 to 9 are utilized and a telephone line network interface 3 of a master station 2 is connected to an end terminal of an analog telephone line network 1. The master station 2 further includes a radio transceiver 4 connected to the telephone line network interface 3 and a controller 5 for controlling the processings of the interface 3 and the radio transceiver 4.
Namely, under the control of the controller 5, a signal transmitted from a central telephone exchange or office side through the telephone line network 1 is received by the telephone line network interface 3 and then modulated by the radio transceiver 4 to obtain a modulated signal of a predetermined channel for radio transmission. The modulated signal is transmitted as a radio signal through an antenna 6 to the first to third remote stations 7 to 9. Each of the first to third remote stations 7 to 9 is provided with a speaker and a microphone for a speech and a dial keypad, etc. The remote stations 7, 8 and 9 receive the radio signal transmitted from the master station 2 by antennas 7a, 8a and 9a connected thereto, respectively, and each remote station has a circuit configuration for discriminating whether or not the radio signal designates itself on the basis of a control signal contained in the radio signal. If the remote station discriminates that the radio signal designates itself, the remote station performs a receiving processing of the radio signal to reproduce a voice signal from its speaker.
Each of the remote stations 7, 8 and 9 detects a voice signal by its microphone and modulates the detected voice signal by a circuit configuration provided therein to obtain a modulated signal for a radio transmission, thereby transmitting the modulated signal from its antennas 7a, 8a or 9a. In the master station 2, the modulated signal transmitted from the remote station is received by the radio transceiver 4 through the antenna 6 and then transmitted to the telephone line network 1 through the telephone line network interface 3. In this manner, a so-called line wire speech can be performed by any of the remote stations 7, 8 and 9 through the master station 2.
Further, each of the remote stations 7 to 9 can transmit a radio signal to the master station with the designation of another remote station, thereby enabling a speech among the respective remote stations, that is, an extension speech.
As one of telephone line networks, a digital line network called ISDN has been started to be serviced. By utilizing the ISDN, various kinds of data can be transmitted effectively and further various controls such as the designation of a terminal receiving an incoming signal or the like can be performed. That is, a desired one of plural terminals commonly connected to one line at the respective subscriber sides can be called by designating the desired terminal through the use of a subaddress or a direct dial-in number (DDI number), etc.
However, in case of connecting a telephone provided in a home to the ISDN, an ISDN telephone is required. That is, in case of connecting the above-described cordless telephone to the ISDN, for example, the telephone line network interface 3 is required to be replaced by the ISDN interface, whereby a speech can be performed through the ISDN at least in each of the remote stations 7 to 9.
However, in case of the cordless telephone, even if the telephone line network interface 3 is replaced by the ISDN interface, there can not get services such as the above-described designation of a terminal capable in the ISDN. That is, unless each of the remote stations 7 to 9 is provided with a circuit for discriminating a subaddress or the DDI number of the ISDN etc., it can not get services such as the designation of a terminal but merely can perform a speech similar to that capable through the use of the normal analog telephone line network, so that the ISDN can not be utilized effectively.
If each of the remote stations is implemented with the ISDN call sequence, the constructions of the remote station become more complicated, then the constructions of a system of the cordless telephone become larger disadvantageously. Further, the similar problem is found when, to the ISDN, there is connected a telephone in which a plurality of remote stations are connected to a master station through signal lines other than the cordless telephone.